Image elimination receiver



March 7, 1939.

` M. G. CROSBY IMAGE ELIMINATION RECIVER Filed July l5, 1957 v 4 Sheets-Sheet l CROSBY Y ATTORNEY March 7, 1939- M. G. CROSBY 2,150,115

v IMAGE ELIMINATION RECEIVER Filed July l5, 1937 4 Sheets-Sheet 2 A [Annu A A A'AAAAAA AL AAAAAA AA lfvvvvvvvvv VVVVVV v I Q99 MMM? l INVENTOR ATTORNEY Marchv 7, 1939.

Filed July l5, 1957 M. G. cRosBY 2,150,115 IMAGE ELIMNATION RECEIVER 4 Sheets-Sheet 3` INVENTOR MU RAYG. @QoS/3)' BY l ATTORNEY March 7., 1939.

M. G. cRosBY v`2,150,115

IMAGE ELIMINATION RECEIVER l Filed July 15, 1957 1&594

4 Sheets-Sheet 4' AAAAAAA VVV,"

INVEN TOR fnac/2035 BY ATTORNEY Patented Mar. 7, 1939 y UNITED; STATES PATENTA orric of Delaware Radio Corporation of America, a corporation Application July 15, 1937, serial No. 153,754

20 -Claims.

means into corresponding amplitude modulaei tion components which are combined with theV inherently resulting amplitude. lmodulation com-- ponent of the combination so that the two types of modulation inherently produced and superposed on the signal modulation oppose and cancel 'on one side of zero beat and aid on the other side. The receiver is tuned so that the desired signal is on the side of the zero beat where the components aid.- This converting system acts as the hetero..v

dyne detector ofy the receiver soV that. the. wave at the output thereof from which the image fre quency has been compensated or suppressedfmay be impressed on a frequency or phase or amplitude modulation wave demodulator.

In a modification, this novel converting means which suppresses the image frequency by opposing components ofv the same on one side of the carrier also reproduces theiuseful signal modulation which can be used directly from the output of the converter. 1

In the prior art of image reduction in superheterodyne receiversl thereductionehas been accomplished by means of selectivity...` The first intermediate frequency is generally made as high as possible so that the radio frequency selectivity will eliminate unwanted signals which are twice the intermediate frequency away from the desired signal. However, high vintermediate frequencies are undesirable because theyicannot be made as selective as the low ones. Consequently, two in-v termediate frequencies are used. on..the more selective receivers so that both-image reduction and selectivity may be obtained.

The arrangement disclosed in this invention is similar in some respects to that of Hansell United States Patent #2,044,745V dated June 16, 1936 wherein abalance is obtained which eliminates the image signal. However, this arrangement applies different principles to produce the balance.

In describing my invention reference will be made to the attached drawings wherein,

Figures 1, 2, and 4 illustrate by circuit diagrams different embodiments of modulated wave energy receivers which include the novel image suppression means described above andarearranged in accordance with my invention. In Figure 4 a heterodyne receiver of the zero beat frequency typel is used whe-reas separate oscillators are used in the other circuits; l.'

Figures 3a to 3h illustrate graphically the manner in which the inherent amplitude modulations and phase modulations of the wave due to the (Cl. Z50-20) local oscillator are related and combined to suppress image frequencies, i. e., frequencies on one side of the carrier, and leave desired frequencies;

.Figure rillustrates a modified lform of receiver wherein a phase modulation detector is used to demodulate thephase modulation component, the amplitude modulation component being limited off; while 1 Figures 6, 7, 8, and 9 are graphs or curves illustrating the operation of the several circuits or ele-ments thereof.

The vector diagrams of Figures 3a, to 3h give a vector representation of the relation between the incoming signal and the local oscillator voltage of `a superheterodyne receiver; Thus, E1 represents the local oscillator Voltage, E2 the incoming signal voltage, and Er the resultant voltage, formed vby the combination of the-se two first voltages. The different representations 3a, 3b, 3c, etc., show successive instances of time. The counter-clockwise direction of rotation is taken as the positive direction; hence as voltage vector E2 rotates counter-clockwise about voltage vector E1, voltage E2 must be higher in frequency than voltage Ei. By viewing the diagrams in the sequence 3a, 3b, 3c, 3d, 3e, 3f, 3d, 3h, voltage vector E2 appears to be the higher frequency and by viewing in theY sequence- 3a, 3h, 3g, 3f, 3e, 3d, 3c, 3b, Vector E2 appears to be the lower frequency.

From the diagrams of Figures 3a to 3h, it can.

be seen that the resultant of the two voltages is modulated. in amplitudeV between the limits given by diagrams 3c and 3g and in phase between the limits given by diagrams 3a and 3e. Thus, there are two'types of modulation present. Ordinarily only the amplitude modulation component is detected to obtain the heterodyne signal, thev phase modulation component being disregarded. ,Howeven lin this invention both of the modulations present are received and combined so that the image, or the signal which is on the other side of Zero beat with the local oscillations, is cancelled out. Careful examination of the vector diagrams in the sequence 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h, and then in the sequence 3a, 3h, 3g, 3b, etc., will disclose that the phase of the amplitude envelope'is 180 apart for the two sequencesl. However, the 'phase deviations are in-phase for the two sequences. Thus, if the detected ampli.- tude modulation were combined with the detected phase modulation` the two buck on one side of Zero beat and aid on the other side. This effects an image elimination. On the other hand there is another point to be considered; that is the fact that the detected phase modulation inherently has a 90 phase shift with respect to the amplitude modulation. To correct for this shift a phase shifter may be placed in a circuit through which the detected phase modulation passes before it is combined with the amplitude modulation. Such a, receiver Would be similar to the receiver of Hansell United States Patent The method used in this invention to combine the two types of modulation is to receive the phase modulation on a frequency modulation receiver. Frequency modulation is the rate of change of phase modulation. The rate of change of the phase Vis 90 displaced from the actual value of the phase. That is when the phase deviation is at its maximum value the frequency deviation is at its minimum value. Thus, such reception inherently applies another 90 phase shift to the detected phase modulation without the insertion Y of a phase shifter and the circuit is simplified. Thus, in my novel system the phase modulation portion of the resultant is received on a frequency modulation receiver circuitV andthe amplitude Vmodulation'portion of the Vresultant is received on an amplitude modulation receiver circuit. 'I'he two resultants are then combined at proper amplitude Vso that the outputs of the two circuits aid for the desiredsignal on one side'of the local oscillator frequency and buck for the undesired signal or image on the other side of the local oscillator frequency.

Figure 1 shows one type of circuit that may be used. The frequency modulation receiver circuit is of the retard circuit pentode detector typeV described in Crosby United States applications #618,154 filed June 20,' 1932, and #25,231 filedV f June 6, 1935. Tube I is a combining tube, 2 the local oscillator and 3 and 4 pentode detectors. The amplitude or Vphase or frequency modulated wave which is to be demodulated with Vimage suppression is impressed by a line connected with kany source such as a wave repeater or an amplifier or anaerial on a circuit 5 tuned to the mean frequency of said wave and from 5 to control grids I2 and I4 of tubes I and 4. The oscillations v produced in 2 of a frequency determined by the constants of 8 are supplied by coupling condenser II to pentode electrodes I6and I8 of tubes I andy low intermediate frequencyY output. If the output .at 1 is of intermediatefrequency a; phase or frequency'or amplitude modulated wave demodulating means may be coupled to 1, depending on Vthe nature of the -wave modulation at 5. If the wave impressed on 5 is continuous the beat'frequency will appear in 1. Y

Y The signal to be heterodyned is fed to tuned circuit 5 and to the grid of tube Ipwhere it is combined with the oscillations fromcircuit 8. The combined voltage is fed from I to transformer or retard circuit 6 and thence to the suppressor 22 of Vpentode detector 3. Theunretarded combined voltage is fed Vdirectly from the plate circuit'of the combiningrtube Irby means of coupling condenser I0 to the control grid20 of tube 3. Thus, pentode detector 3 has retarded voltage fedto one grid, 22, and unretarded voltage fed to the other grid, 2B, and the detected frequency Vor phase modulation will appear in its plate circuit. If we call the time delay of the wave produced in circuit 6 "D the phase of the output of this circuit would be: Y

a: 2r?. n (1) Thus', the phase of the output of the retard circuit 6 is directly proportionalto the applied frequency. In the case of a frequency modulated wave given by:

e=E cos (wt-fd/fm cos Vpt) (3)V where w=21r carrier frequency,` fd=frequencyV deviation with modulation and p=21rfm, where Y fm=the modulating frequency, the instantaneous frequency is given .by Y Y I d(Wt-fd/fr cos pt dt t= (fo-l-fd sin pt) (5)v Substituting (5) in (2) gives:

a=21rD(fc+vfd sin pt) (6) Hence, the wave passed through the retardationV `circuit has the phase given by (6) added toit, or

The grids of the detector 3 are adjusted according to the characteristics of Figures 6 and '7. Both grids are adjusted to point "a which is the linear portion of the characteristic. Thus, in accordance with the general detailed description of this detector given, in my United States appli-V j cation Serial #716,469 filed March 20, 1934Pat-- ent #2,063,588 dated December 8, 1936, thevariable current inthe output of the detector would be substantially rgiven by: Y

' Y J=l1 Y1.2: e1 e2 t `(8) where ai and az are constants of the tube and e1 and ez are the two grid voltages. Applying the unretarded voltage of Equation (3) to one grid, say 20, andthe retarded voltage of (7) to the other grid, say 22, gives as the variable output:

simplifying and eliminating radio frequency terms which would be eliminated by the low pass filter in the detector output, the variable output Y would be:

J=hzos 21rDfc cos 21rDfd sin ptsin 21rDfc sin 21rDt'd sin pf} (10) In practice, 21rDfc is adjusted to equal 1r/2, 31r/2, 51r/2, etc., so that Y aizEiEz J= 2 sm 21rDfd sm ptY (11) by applying the Bessel Function Expansion:V sin(Xsin)= ,Y

2.11 (X) sin +2Ja (X) sin 3+. (12) the following is obtained:

J= @ma Dfd) sin pf# Y 1 ]3(21rDfd) sin pt+. .l (13) Thus, the output of the detector 3 consists of the fundamental frequency sin ,pt :proportional The. amplitude .modulation component of. `the resultant of the local oscillator and signal is detected by means of pentode detector tube 4. Oscillations from local oscillator 2 is fed to one grid I 8 and the signal to the other grid I4 and the detected amplitude modulation appears in the platel circuit in the conventional manner known to the art. Thus, the detected amplitude component andthe frequency detected phase component obtained as described above are combined in transformer I and the heterodyne output is taken from the secondary of this transformer. Moreover, as pointed out in detail hereinbefore, the components inherently produced by amplitude modulation of the signal Wave and phase modulation of the signal Wave `by the oscillations from oscillator 2 buck out on one side of Zero beat and add on the other side of Zero beat to produce the signal and eliminate image frequencies. This operation will be clearwhen one keeps in mind the fact that as indicated in Figures 3a to 3h the polarity of the envelope of the amplitude modulation component is opposite on both sides of zero .beatwhereasthe envelopes of the phase modulation 'component arethe same on both sides ofzero beat.

In the circuit of Figure 2the slopinglter type of frequency modulation receiver is employed. The Wave energy is fed from circuit li to grids I2 and I4. oscillations are fed from oscillator tol the control grid 34 of amplier and coupling tube 9 and to the pentode electrode I8 of detector 4. The output of tube l is beat With the output of tube S and the energy of intermediate frequency is impressed on a filter 5 having a sloping characteristic as illustrated in Figure 8. This lter converts the phase or frequency modulations on the Wave into amplitude changes which are impressed on the 'control grid 2!) of detector 3. rllhe output of detector 3 contains modulation components characteristic of signal modulation by the combination of oscillations from oscillator 2 and coupling tube 9 and the signal Wave.

4rllhe Wave energyis also impressed from circuit 5 on grid I4 of tube 4 and from oscillator tube2 and tuned circuit 8` through coupling condenser Il to the electrode I8 of tube 4 so that the inherent amplitude modulations by the local oscillator 2 are detected in tube 4 as in Figure l. Separate combiningl tubes Ir and S combine the local oscillations from oscillator and circuit 2 and 8 and the. signal from circuit EV so that their resultant appears in the common plate. circuits including iilter 6 of the combining tubes I and 9. This resultant is fed through sloping filter B and thence to the control grid 2t of detector 3y to detect the phase modulation component resulting from beating local oscillations from tube 2 with the received Wave from circuit 5. Pentode detector tube i detects the amplitude component in the same manner as in the circuit of Figure 1.

The two energies resulting from detecting `these components arecombined so Ythat signals on one side of the iocfai emulator adcand those on the other side thereof buck Vand .compensate or neutralize. If .wave energies onone side of the local oscillator add they must oppose on the other side of the local oscillator. The output or coupling transformer l which is free of image frequencies may beimpressedon any utilization circuit such as an` intermediate frequency amplifier or an audio frequency amplifier for beat note reception.

In the above circuits the manner in Which element voltages are applied hasbeen omitted for simplicity inasmuch as their placement and values Will be obvious to one versed in the art from the illustration thereof. Automatic volume control circuits may be added in conventional manners in order to adjust the balance and the operating levels.

The operation of the above circuits is not confined to any frequency. Thus, the principle may be applied to a first or second detector of a superheterodyne or may be` applied to an autodyne or non-superheterodyne receiver. Figure 4 shows the principle applied to an autodyne receiver. Autodyne ampliiier tube 36 with its input circuit 5-andregeneration circuit 38 supplies the combination of local oscillations and signal to be detected as amplitude modulation in detector tube andas phase or frequency modulation in frequency modulation detectorY 3t The phase modulation component is demodulated by passing the.. combined signal and oscillator voltage through coupling or limiter tube 5i! and, converting to amplitude modulation by means of the side of the resonance of tuned circuit 6 which has a characteristic as shown in Figure 8. Tuned circuit t may be replaced by any type of sloping lter such as shown at 6 in Figure 2. If tube Mlis a limiter tube it helps to remove the amplitude modulation component so that detector tube 3 produces an output purely dependent upon the phase modulation component of the resultant voltage. However, such. limiting is not strictly necessary since the amplitude component appearing in the output of detector 3 may be balanced out by energy ofopposite phase from the amplitude modulation component supplied by detector tube 4. The outputs of the two detectors are combined in proper phase, as determined by switch 3U, so as to add the components on a selected side of the local oscillations and oppose on the other side of the local oscillations to balance the image signal. Adjustment of the balance could be made by varying tuned circuit 6" or by varying a volume control in a circuit supplying voltages to an element ofl tube 4t, such as control grid 42.

4By eliminating tube 4 of Figure 4 and utilizing the output of detector tube 3 only, a novel form of detection Would be available which would have distinct advantages. This type of detection would be the detection of the phase modulation component of the resultant of the combined local oscillator and signal voltages. Ordinarily only the amplitude modulation component is detected. However, this receiver would limit off the amplitude modulation component and receive the phase modulation component as frequency modulation. Such a detection makes the heterodyne output proportional to its frequency. Hence, as the heterodyne note is made closer to zero frequency its output Will be decreased towards zero. This enables the elimination of certain interfering signals by tuning the heterodyne note of the desired signal so that the interfering signal is heter--4 element voltages to cause limiting. `I'he limiter vlill removes the amplitude modulation component of the combined energy and leaves the phase modulation for Vdetection in pentode detector tube 44. Detector tube 44Y acts in the manner described ln Crosby United States application #716,469 led March 20, 1934 and detects the phase modulation by having the phase modulated energy applied to one grid, G1, and the carrier from oscillator 2- on theV other grid, G2. Proper phase adjustment can Y be made by varying the tuning of circuits 6 and 42K. The manner in which demodulation of vthe limited phase modulated wavein detector tube 44 rwill now be described. The limited energy, ad-

justed in phaseby circuits 6" and 42, is applied to the control grid Grof detector tube 44. Oscillatory energy Vis supplied by oscillator 2 to the l pentode electrode G2 of detector tube 44'.

In the explanation which follows, it will be `assumed that the signal on G1 is modulated in amplitude ,and'phase t Assuming a signal modulated in both amplitude and phase given by e=E1(1-1m sin pt) sin (Wfl-qb sin qt) V(14) VVwhere m is the percentage amplitude modulation,

'fp the angular velocity ofthe amplitude modula-V tion wave, p the depth of 'phase modulation, lq

the angular velocity of the phase modulatingV .wave, and w is the angular velocity of the carrier wave.

Figure 9 shows a static characteristic of either of the grids of the multi-grid detector of this invention.Y That is, if either grid voltage is varied according to the abscissa of Figure 9, the plate current will vary according to such a curve.

Hence, the overall characteristic of the varyingv current in the output due to the two grids will be represented by: n

i=(k1E/|a1e1+b1e2f+C1e13+- -XkEzH-l' 2e2+b2e22+c2923+ Y (15) where E and E" arerthe'constant grid biases of the two grids, e1 and e2 are the alternating current voltages applied to the two grids and k1, k2, 0,1, a2, etc., are constants depending upon the tubef l Multiplying (l5) out gives:

Since the tubes are adjusted so that a1 and a2 are large compared to b1, c1 and b2, cz, etc., the majority of the output current is given by:

after the permanent direct current and radio requency terms have been eliminated.

In the receivers of the type portrayed in `Eigure 5, the alternating current voltage to one grid is given by Equation (14) and the voltage to the Vportional to J1 (qi).

other grid is either the filtered carrier or a locally synchronized carrier given by:

Y e2=E2 sin (Wt-l-) Hence, the detector output would be:

sin t) sin (m4-B) (19) which, simplified andwith radio frequency terms eliminated, reduces to: .Y l

(l-I-m sin pt)[cos sin qt) cos B+' sin (QS sin qt) sin B] (20) Applying the Bessel Function expansions transforms (20) to 2.11015) COS 4qf') COS B+(211() sin qH- V ivd. c.

When it is desired to receive the amplitude modulation component, the local carrier phaseV is adjusted so that B is 0 or 180, 360, etc., degrees. The alternating current output will then When it is Vdesired to receive phase modulation B is adjusted to 9() or 270, etc., so that Equation indicates that when phase modu` lation is being applied from circuit 42 to the grid G1 the detector output contains: 1. The desired phase modulation fundamental output. 2. The odd harmonics of the phase modulation. 3, The beats between the amplitude modulation fundamentalV and the phase modulation fundamental and odd harmonics. 'I'he phase modulation fundamental output is unaffected Vby the presence of the amplitude modulation component and is pro- In the absence of amplitude modulation, only odd harmonics of the phase modulation are present in the detector output.

Thus, when `phase 'modulation is fed in on G1 from circuit 42 and carrier modulation is fed in on G2 from oscillator tube 2 the fundamental modulation frequency, which is the diierence frequency between the signal waves and those from oscillator tube 2, is detected by detector tube 44 in the manner described above. Thus, the difference frequency appears in transformer 1 by virtue of a detection of the phase Vmodulaiter.

tion component present onthe resultant ,in circuit 42.

In the .apparatus of Figure 5, tube B0 is alim- Consequently, it will remove the amplitude modulation component of the combination ofthe local oscillator 2 and the incoming signal.

`This will-rleave the phase modulation of 'the resultant as the only'modulation left for the.Y purpose of detecting the beat betweerr the local oscillator and the incoming signal. Since phase modulation cannot be detected by' the ordinary nonlinear device, a detector 44 of the multigrid type is employed and the phase modulated signal is applied to G1 while theA local oscillator i's fed to G2. Thistype of detection detects the phase modulations ofthe resultant just as if the phase-modulation were converted to amplitude modulation first andY detection then accomplished on a nonlinear device. -The above mathematics which result in Equation (25) show the output of such a detectorwhen a Waveis applied at its input which is both 'amplitude and phase modulated. Due to the action of the limiter tube 40, the amplitude modulation component will be removed malcing -m=zero. rFne resultant output will then begiven by:

v which is equal to the differencey between the local oscillator frequency and the incoming signal frequency. This frequency is the desired beat frequency and is the samefrequency that would be obtained if the incoming signal and the-local oscillator were combined and detected for the amplitude modulation presenton their resultant in the conventional manner.

In any of the above describedy circuits Vthe Wave may be frequency multiplied at any pointl Where the modulation present is substantially'phase modulation. This would increase the phase deviation of the resultant of the incoming wave and the local oscillator. For instance, in the-circuit of Figure 1, retard circuit 6 might'be tuned to twice the frequency of the incoming signal so as Y tomake tube l a frequency multiplier. In Figure 2, circuit 6 could be tuned inthe same manner so as tov make tubes I and 9 frequency multipliers. In Figures 4 and 5, tuned circuits 6" and 42 may also be4 tuned to a harmonic to make tubes 40 frequency multipliers. This application of frequency multiplication would increase the sensitivity of the detecting system. Limiting might also be accomplished in amplifier tube I of Figures 1 and 2 as well as in coupling tubes 40 of Figures 4 and 5.

I claim:

1. The method of suppressing the effects of an undesired band of frequencies Which includes the steps of, receiving signal energy including said undesired band of frequencies, generating local oscillatory energy, beating the undesired band of frequencies With the locallyA generated energy to produce resultant energy having phase `medulation and amplitude modulation Mcomponents thereony resulting fromfsaid beating action, de-

tecting said components and simultaneously producing relative phase displacement in the detected energies and combining the detected energies to balance out the beat frequencies characteristic of said undesired band of frequencies.

2. The method of suppressing the eects of an Y undesired band of frequencies which includes the steps of, receiving signal energy including said rand combining the resultant energy to balance out the beat frequencies characteristic of said undesired band of frequencies.

3.l The method of suppressing the effects` of undesired bands of frequencies on receivers of the heterodyne type which includes the steps of receiving signal energy including said undesired band of frequencies, generating local energy, beating the undesired bands of frequencies with the locally generated energy to produce beat frequencieshaving phase modulation and amplitude modulation components thereon resulting from said beating action, separately detecting said componentsI and simultaneously producing rela.- tive phase displacement between the detected components and directly combining the resultant energy to balance out the beat frequencies characteristic of said undesired band of frequencies.

4. 'Ihe method of receiving desired bands of frequencies and excluding undesired bands of frequencies which includes the steps of receiving all of said bands of frequencies, generating local oscillatory energy differing from said received energy by a beat frequency, beating said local oscillatory energy with all of said received energies to produce a beat note modulated in phase and amplitude in accordance with said undesired frequencies, demodulating the phase modulated component of said beat note, and simultaneously producing a phase shift in the demodulation component, demodulating the amplitude modulation component of said beat note, and combining the demodulated energies whereby the energies characteristic of the undesired bands oppose and the energies characteristic of the desired bands add.

5. The method of receiving desired bands of frequencies and excluding undesired bands of frequencies which includes the steps of receiving c all of said bands of frequencies, generating local oscillatory energy differing from said received energy by a beat frequency, beatingy said local oscillatory energy With all of said receivedY energies to produce a beat note energy modulated in add.

6. The method of demodulating desired signal carrying Wave energy and undesired signal modulated Wave energies of different frequencies and eliminating from the output demodulation com-Y ponents characteristic of the undesired wave energies which includes the steps of, receiving all of said Wave energies, generating local oscillations differing from said received Wave energies by a beat frequency, combining'all of said wave energies with said locally generated oscillations to produce energy of beat frequency which is modulated in phase and in amplitude by said combining action, separately demodulating the phase modulation components on said energy of beat frequency and simultaneously producing a phaseY shift in the demodulated energy relative to the phaseV modulations on said energy of beat frequency, demodulating energy characteristic of 4 the amplitude Amodulations on said energy of beat frequency, and combining said last demodulated energy with said first demodulated energy whereby the demodulated energies on one side of zero beat frequency adds and the demodulated energies on the other side of zero beat frequency oppose and cancel.

7. In a systemV for receiving wave energy modulated at signal frequency and undesired wave energy and for demodulating the same and removing the effects of the received undesired Wave energy on the modulated output including, Wave receiving means of the heterodyne type, a utilization circuit, -a frequency modulated Wave demodulating means connecting said first means to said utilization circuit and amplitude modulated Wave demodulating means of the heterodyne ,type coupling said first means to said utilization circuit.Y Y,

8. In a'system for demodulating Wave energy and suppressing image frequencies caused by rel, ceived undesired Wave energy, a combining tube having input electrodes excited by all of said wave energies, said combining tube having output electrodes, a source of oscillations of a frequency different than the frequency of said received wave energy, means for impressing oscillations from said source on said vcombining tube to beat with said'received Wave energy, a first Ydischarge tube. I 1 Y 9. In a system for demodulating Wave kenergy and ,suppressing image frequencies caused by received undesired wave energy, a combining tube having input electrodes excited by all ofVV said Wave energies, said combining tube having output electrodes, a source of oscillations of a frequency different than the frequency of said t received Wave energy, tube means for impressing oscillations from said source on said combining tube to beat'with said received wave energya rst electron discharge tube having input and output electrodes, a frequency discriminating circuit connecting the input electrodes of said first electron discharge tube tothe output electrodes of'said combining tube, a second electron discharge tube having electrodes excited by'said YWave energy, said tube having output electrodes; tube means coupling the input electrodes of said second electron discharge tube'to said oscillator,V

'and a 'common circuit connected withthe output electrodes of said first and second dischargetube. 10. In a'systemY for demodulating wave energy and suppressing image frequencies caused vby received undesiredwwave energy, an oscillatingY combining tube, a second electron discharge' tubev Yhaving input and 'output electrodes, a circuit Y coupling the input electrodes of said second discharge tube to said oscillating combining tube, and a common circuit connected withzthe output electrodes of said first and second discharge tube.

11. A system as recited in claim Y8 including means for tuning said frequency discriminating circuit to a harmonic of the frequency of the Wave energies, and means for causingr said combining tube to act as a frequency multiplier.

l2. A system as recited in claim 9 including means for tuning said frequency discriminating circuit to a frequency which is a*harmonic of said Wave energy. Y

13. A system as recited in claim 10 including means for tuning said frequency discriminating circuit to a `harmonic of said wave energy.y

14. The method of demodulating Wave energy modulated in phase, frequency, `r amplitude and for eliminating `from the resultant energy image frequencies caused by undesired Wave energy of a frequency adjacent to the frequency ofthe desired Wavev energy Vwhich includes the steps of, beating all of said'wave energy with locally generated oscillations differing from said wave energies by an intermediate frequency, said beating action lproducing intermediate frequency en- `ergy modulated inV phase and in amplitude Vdue `modulated in phase, frequency, or amplitude and for eliminating from the resultant energy image frequencies caused by undesired wave energy of a frequency adjacent to the frequency of theV desired Wave energy which includes the steps of,

beating all Vof said Wave energy with locally:

generated oscillations differing from said wave energies by an intermediate frequency, said beatingaaction producingan intermediate frequency energy modulated in phase Aand in amplitude due to the combination of said Wave energiesrwith said locally generated oscillations, demodulating e the phase modulations on said intermediate fre-V quency energy' and simultaneously producing a 90 phaseY shift in the demodulated energy relative to the phase modulations on saidintermediate frequency energy, Vdemodulating the amplitude modulations on said intermediate frequency energy, and combining theresultant er1-1Y ergies whereby the modulations Yresulting from `'lli the desired Wave energy add and the modulations resulting from the undesired wave energy oppose and cancel.

16. The method of demodulating modulated Wave energy Which includes the steps of, combining said modulated energy with locally generated oscillations to produce resultant energy, limiting and frequency multiplying the resultant energy, and detecting the phase deviations produced on said resultant by said combination to produce signal carrying energy of a frequency equal to the difference between the frequency of said locally generated oscillations and the mean frequency of said modulated energy.

1'7. The method of demodulating modulated Wave energy which includes the steps of, combining said modulated wave energy with locally generated oscillations to produce resultant energy the phase of which varies due to said combination, limiting and frequency multiplying said resultant energy,l and detecting the effective frequency variations of the said phase variations of the resultant energy.

18. The method of demodulating modulated wave energy which includes the steps of, bombining said modulated Wave energy with locally generated oscillations to produce resultant energy varying in phase due to. said combination, limiting and frequency multiplying the resultant energy, detecting said phase variations on said resultant energy to produce energy of a frequency equal to the difference between the frequency of said locally generated oscillations and the mean frequency of said modulated energy, and demodulating said energy of difference frequency to reproduce the signal.

19. The method of demodulating modulated wave energy which includes the steps of, combining said modulated energy with locally generated oscillations to produce resultant energy varying in amplitude and in phase, limiting the resultant energy to substantially remove said Variations in amplitude, and detecting the phase deviations produced on said resultant by said combination to produce signal carrying energy of a frequency equal to the difference between the frequency of said locally generated oscillations and the mean frequency of said modulated energy.

20. The method of demodulating modulated Wave energy which includes the steps of combining said modulated Wave energy with locally generated oscillations to produce resultant energy varying in amplitude and in phase due to said combination, limiting the resultant energy to substantially remove therefrom the said variations in amplitude, detecting said phase variations on said limited resultant energy to produce energy of a frequency equal to the difference between the frequency of said locally generated oscillations and the mean frequency of said modulated energy, and demodulating said energy of difference frequency to reproduce the signal.

MURRAY G. CROSBY. 

